The present invention relates to a porous hollow fiber of a polysulfone suitable for use in filtration, e.g. microfiltration. More particularly, the invention relates to a polysulfone porous hollow fiber which has a relatively large pore size, and the surface fraction of openings on the inner or outer surface of which is extremely increased by providing the inner surface with a structure such that approximately circular open pores (openings) are formed at intervals not exceeding the average diameter of the openings or by fibrillation of the outer surface portion to a high degree, whereby the clogging is prevented and the filtration life is prolonged.
Polysulfone porous hollow fibers have been appreciated to have excellent heat resistance and chemical resistance and have been widely used for industrial microfiltration, ultrafiltration and other purposes.
Conventional polysulfone porous hollow fibers have been prepared by so-called dry-wet spinning method which basically comprises extruding a homogeneous or approximately homogeneous raw spinning solution of polysulfone together with a core liquid into the air through a double ring nozzle and then immersing in a coagulation liquid.
As the dry-wet spinning method, there are proposed many methods wherein the composition of the polysulfone solution, the composition of the core liquid, the composition of the coagulation liquid and the spinning conditions such as temperature and spinning speed are altered and combined. For example, as the polysulfone solution, there is used a solution of polysulfone dissolved in a mixed solvent of an aprotic good solvent such as dimethylformamide, dimethylacetoamide or N-methyl-2-pyrrolidone and a non-solvent such as propylene glycol, which causes a phase separation at a temperature below room temperature (as disclosed in Japanese Patent Publication Kokai No. 60-222112), a polysulfone solution to which polyethylene glycol having a molecular weight of 400 to 20,000 is added as an additive (as disclosed in Japanese Patent Publication Kokai Nos. 57-35906 and 58-Japanese 114702), or a polysulfone solution to which polyvinyl pyrrolidone having a molecular weight of 10,000 to 450,000 is added as an additive (as disclosed in Japanese Patent Publication Kokai Nos. 61-93801, 61-238306 and 63-97205). Hollow fibers having openings with a diameter on sub-micrometer order are obtained by extruding such a spinning solution with a core liquid, e.g. an aqueous solution of the above-mentioned good solvent consisting, a mixed solvent of a good solvent and a non-solvent or mixtures of these core liquids and the above-mentioned additives such as polyethylene glycol and polyvinyl pyrrolidone, through a double ring nozzle at a temperature such that the spinning solution is kept homogeneous or approximately homogeneous, running the extrudate several centimeters to several tens of centimeters in the air and coagulating it in water or an aqueous solution of a good solvent.
In general, clogging of filtration membranes is considered to result from the mechanism that impermeable substances are deposited on the surface, and the mechanism that impermeable substances are caught and accumulate inside the filter medium. In the former case, as would be easily understood, the larger the surface fraction of openings on the surface of the filtration membrane, the harder the clogging occurs and the longer the life. In the latter case, too, it is apparent that a larger surface fraction of openings on the membrane surface is desirable so long as impermeable substances have a uniform size and the life with respect to clogging does not depend on only the amount of impermeable caught and accumulated inside the membrane. Evaluating conventional hollow fibers produced by the above-mentioned method from such a point of view, the inner surface has a surface fraction of openings of about 10-40% and openings of circular shape or irregular shapes are formed, and the outer surface has a surface fraction of openings of at most several tens of % and openings of circular or irregular shapes are formed. It has been pointed out that conventional hollow S fibers are insufficient in life in both the uses where the filtration is conducted in the direction toward the outer surface of the hollow fiber from the inner surface and the uses where the filtration is conducted in the direction toward the inner surface of the hollow fiber from the outer surface.
It is expected that the clogging is reduced by increasing the surface fraction of openings on the inner and outer surfaces of the hollow fiber, thus the life could be prolonged. However, hollow fibers having both a sufficiently high surface fraction of openings and a sufficiently high strength cannot be obtained by a method according to conventional methods. For example, even if a known spinning solution is spun using a core liquid having a lower coagulating function, an inner surface having a structure which is composed of irregular openings and continuously distributed spherical polymer portion, is formed. The surface fraction of openings does not exceed 50%.
It is an object of the present invention to provide a polysulfone porous hollow fiber having an extremely prolonged life with respect to clogging of pores of hollow fiber as compared with conventional polysulfone hollow fibers.
Another object of the present invention is to provide a polysulfone hollow fiber which has a large surface fraction of openings and can be used in filtration such as microfiltration or ultrafiltration for a long term.
These and other objects of the present invention will become apparent from the description hereinafter.